Electric capacitor



May 17, 1938. F. M. cLARK ELECTRIC CPACITOR Filed Jan. 27, 1934 Iventor:

1 w CM. w M tp m W a w H W Patented May 17, "1938 PATENT OFFICE ELECTRICCAPACITOR Frank M. Clark, Pittsfleld, Mass., assignor to .GeneralElectric Company9 a corporation of New York Application January 27,1934, Serial No. 708,621

6 Claims.

The present invention relates to electric capacitors and is moreparticularly concerned with improved capacitors employing armatures orelectrodes consisting of unoxidized material which, however, is capableof forming dielectric films thereon, these armatures being separated bya thin dense, relatively non-absorbent dielectric septum, and containinga non-aqueous liquid having dissolved therein an ionogen.

One of the principal objects of my invention is to provide capacitorswhich do not require filming of the electrodes before use. It is anotherobject of my invention to provide a stable capacitor of high efiiciencywhich will not deteriorate during periods of idleness.

It has been proposed, heretofore, to employ as .an electrolyte in acapacitor or rectifier a solution of a suitable salt in a water-freepolyhydric alcohol. Porous absorbent material has also been used as asupporting means for the electrolyte in capacitors. Such materialtdoesnot constitute a dielectric septum but functions rather as a support forthe electrolyte. The electrodes employed in this type of capacitor aregenerally made of aluminum and are known as the film type" electrodes.By "film" is meant a dielectric layer consisting of oxide or hydroxide,which is formed on certain metals when subjected to a chemical processor to electrolysis in an aqueous solution. Such films may be prepared onthe armatures prior to or after assembly of the apparatus by a specialtreatment which adds to the cost of manufacturing such articles.Furthermore, the employment of such '*filmed electrodes makes itnecessary to put the capacitors containing them into service promptlyand to maintain them in service without long interruptions as the filmdeteriorates when the device is not used. w

I have discovered that a non-aqueous alcoholic solution of Rochelle saltcan be used with marked advantages in connection with unoxidized,potentially film-forming electrodes or armatures, such as aluminum ortantalum electrodes, and in combination with a substantially non-porousdiaphragm of dielectric material such as paper, cellulose esters (suchas the acetate), cellulose ethers (such as the methyl or ethyl ether)sheet mica, or varnished'silk, or other septum of dielectric materialwhich is chemically inert with respect to such solution. Suchsubstantially non-porous, dense sheet dielectric is not used merely as aspacer for the absorption of impregnant, but serves itself in adielectric capacity.`

While the armatures or electrodes may become filmed during use, theoperativeness of the capacitor does not depend on the presence of suchfilms. During periods of non-use the electrode films may deteriorate butthe presence of the dielectric septum allows the capacitor to be putinto immediate use, dispensing with the special refilming of theelectrodes.

The use of previously chemically oxidized aluminum foil in my capacitorwould deorease the capacity and increase the power factor valueapproximately 4% times as compared to the use of non-oxidized aluminumfoil.

The use of a porous absorbent material, such as cotton cheesecloth,linen cloth or the like in capacitors in place of the dense sheetmembrane of non-conducting material also causes an increase of powerfactor or energy loss. For example, the introduction of cheesecloth incapacitors embodying my invention raises the power factor from 8 to 18per cent.

Capacitors embodying my invention have extraordinary high capacities fora given armature (electrode) surface. They have extremely small bulk,being assembled in a compressed state with the armatures separated onlyby the thickness of the paper or other dielectric septum. As alreadyindicated, no "forming of the electrodes is required and substantiallyno deterioration results in periods of idleness. i

While I employ a solution of a salt in my capacitor it does not have allthe properties ordinarily characteristic of an electrolytic capacitor.The introduction of a dielectric septum causes the capacity to drop asthe thickness of the dielectric material increases.

In the accompanying drawing Fig. 1 shows in side elevation a partlyunrolled capacitor assembly which is provided with a paper dielectricmember and Fig. 2 shows in side elevation a complete capacitor in itscase which contains a liquid composition made in accordance with myinvention.

The electrodes of the capacitor shown in Fig. 1 consists of sheets I, 2of unoxidized aluminum, tantalum, or other suitable potentiallyoxidizable metal. Adjacent the sheets and 2 are sheets 3, 4 ofsubstantially non-porous dielectric material. When the sheets thusassembled are wound up the sheets of dielectric 3 and 4 are intcrposedbetween successive layers of metal foil I and 2. Instead of a singlesheet of paper it is preferable in some cases to employ two or eventhree sheets, but in the drawing a single sheet has been shown to avoidcomplicating the drawing. I prefer to employ kraft paper such asdescribed (Fill in United States Letters Patent l,850,7ll2 issued March22, 1932, to Amos Allen. Such paper has' a density at least as high asabout W95. The other materials mentioned above as suitable for use inplace of paper have still higher density. Contact to the metal foilwhich constitutes the armatures of the capacitor is made by stripconductors 5,

The capacitor assembly is subjected to the heated liquid impregnatingmaterial for at least about one-half hour at a temperature of ll0 C. oreven higher. As already indlcated above, Rochelle salt is dissolved inalcohol for the preparation of the treating solution. As the alcohol Iprefer to omploy ethylene glycol, althoush glycerine, or otherpolyhydric alcohol in which the ionogen is soluble to the olesiredamount may be employed. The alcohol is distilled in a vacuum under'conditions suitable for removing water vapor and other impurities.Rochelle salt which, as well known, is a tartrate of sodiurn andpotassium, pi'eferably should be dissolved in the crystalline state,that is, without removing the water of crystallization. The alcoholshould be employed as a saturated or nearly satlu'ated solution. About 5to lo per cent by weight of the Rochelle salt is present in a saturatedsolution of this salt in ethylene glycol. Qrdiriarily about one to tenper cent of the Rochelle salt ionogen should be present in the solution.

After treatment in this solution the capacltor assemblies arehermetically sealed into containers 'with suitable sealing compouhds,such. ;7 exe ample as ll/lontan Wax, asphalts, or resinous con--positions. Fig. 2 is showh a distinct body of treatine' liquid llsurrounding the capacitor as-=- semloly but it should be understood thatthe as seinbly rnay substahtially lll the cashig 9, the liquid merelywetting and adherlng to the capacitor surfaces. rllhe conductors 5, Ciare connected to the external contacts U0, M.

The high capacity of a capacitor enibodylng my present invention may bemade evldent a comparison of the same with a capacltor made up similarlyof liraft paper treated with wasi or mineral'oil. In the case of acapacitor made up in accordance with my invention approxirhately .001square foot of active armature or' oll suzface will have a capacity ofoue mlcrofarad. (Oh the other hand, a wax or oil treated capacitorhaving the same construction requires a oll or armature surface of about8 to ll) active square feet for the same capacity of one microfarad. A.capacitor, such as shown in the drawlns, employing as armatures -f'fmil. aluminum roll having a length of 1% feet and a width of 31/4.inches separated by two or three sheets off ;fe mil. lrait paper andtreated -as above descrlbed a capacity of 700 to Boli microfarads. Thepower factor of a capacitor made in accordance With. my invention varleswith the voltage. At l to volts per unit the poweractor is about lll percent. At higher voltagesthe power factor ln= creases, rising to 15 percent at 8 volts and 18 per cent at 20 volts.

As the power actor is a measure of energy loss it is desirable to employan arrangement which will keep down the power lactor to a miminum andtherefore I prefer to connect suicient units of capacity in series'tokeep the voltage applied to each unit within the llmits of commerciallysuitable power factor values. For eitample, when it is desired toconnect a capacitor made in accorclance with the present invention toapparatus Operating at volts, I prefer to .pregnating' liquidconsisting' of ethylene glycol connect 15 units in series, each unittherelore Operating at approximately 8 volts and givlng a power actor ofabout 15 per cent. While with such a series connection the highefficienoy of capacity will be reduced it is still high as com- 5 paredwith war: or oil treated capacitors, for example with a capacity ofabout 50 microfarads approximately 25 square feet of active foil sur-aface are required When Operating with a series connection of 15 units.In a Wax treated capac- 10 itor which will operate directly on llllvolts a single unit about 400 square feet of active foil surface arerequired for a capacity of 50 microarads.

Capacitors embodyiue1 my invention are espeu 15 cially well suited iorintermittent duty voltages, that is, voltages of llll volts or higher orfor continuous duty at low voltages.

Capacitors employlng lzraift paper and an w and Rochelle salts whenconnected in units, El series and tested llll volts intermittc" showedgreat stability. N'filter llldlllll repetitlons of voltage thecapacitcrshave been found substantlally intact, the power factorshowing' only 513 a slight rise.

l do hot wish to be urlderstood as lina eizact construction o escribe l.i

ited the as lie various cations ot the sanle "ring withih the ue spiritand scope of vention be covered in the Yfehded clalwls.

En. an apphcation, Serial No. llf,

o; capacitors similar to the capacitors covere the claiins hereolcontaining ammonlu borate in place of Rochelle salt.

What l claim new and desire to secure by Letters of the United ltates,is:

l. An electric capacltor comp'rising the c bihation of jusztaposeduhozlfllzed armatures, a non-aoueous solu on of salt in ethylene glycollocated in armatures, and at least che Sheet of sub" stantiallynon-porous located between armatures and substantially insrdatlna' the sfrom one another, said paper havinc' of at least about Glfil.

!ln electric capacitor comprlslhlnatioh of jusztaposed unoridized :farmatures, a non-adueous solution consists at least 5 per cent ofRochelle sahts in a polye hydi'ic alcohol in contact with saidarlhatui'es,

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and a substantially non-pointu dielectrlc septu a varmatui'es to vaiyinversely with the thiclmess of said septum and to permit said capacitorto operate without film-formation.

4. An electric capacitor comprising the combination of non-oxidizedmuminum armatures assembled with interposed non-conducting sheetmaterial in a compressed state, said sheet mate- 75 rial consisting ofone or `more sheets of kraft paper having a density of at least about0.95 and a solution of Rochelle salt in substantially waterfreepolyhydric alcohol filling voids between said armatures.

5. An electric capacitor comprising the combination of juxtaposed,unoxidized, illm-forming capacitor armatures and a liquid non-aqueousmaterial consisting ofv polyhydric alcohol and Rochelle salt located incontact with said arma- .tures and one 0x' more sheets of substantiallynon-porous dielectric material located between said armatures, thecombination of said dielectric material and said liquid resulting in acapacity which is materially higher than the capacity of' a wax oroil-treated capacitor and which varies inversely 'with the thickness ofsaid dielectric material.

6. An electric capacitor comprising the combination of juxtaposedaluminum armatures, a solution containing at least 90 per cent ofethylene glycol and containing Rochelle salts located in contact withsaid armatures and a non-poros septum of substantially non-conductingsheet material located between said armatures, said material having asufllciently high dielectric property to cause the capacity between saidarmatures to vary inversely with the thickness of said sheet material.

FRANK M. CLARK.

